    Patent Document 1: Japanese Unexamined Patent Publication (kokai) No. Hei 2-102912 discloses a bearing apparatus, in which a heat-treated S45C shaft (hardness HRc 55) and an aluminum-alloy plain bearing are combined.    Patent Document 2: Japanese Unexamined Patent Publication (kokai) No. Hei 2-57653 relates to a bearing apparatus, in which a heat-treated steel shaft (S45C, hardness HRc 55) and an aluminum-alloy plain bearing are combined. The aluminum alloy is Al—Sn based and contains, by weight percentage, 7 to 20% Sn, 0.1 to 5% Pb, 1 to 10% Si, 0.3 to 3.0% in total of one or more of Cu, Mg and Zn, and 0.01 to 1.0% in total of one or more of Cr, Mn, Fe, Ni, Co, Ti, V and Zr, with the balance essentially consisting of Al. In the Al—Sn based bearing alloy, 0.01 to 0.1% Sb is added to strengthen the matrix and to modify the shape of the dispersed and precipitated Si particles in the Al matrix to such forms as spheroid, oval, or those having rounded ends. In the case of end-rounded Si particles according to Patent Document 2, an ideal point contact is realized. The hardness of the alloy is about Hv 40.    Patent Document 3: Japanese Unexamined Patent Publication (kokai) No. Hei 6-235038 also relates to a bearing apparatus, in which a heat-treated steel shaft (S45C, HRc=55), and an aluminum-alloy plain bearing are combined. The aluminum alloy is Al—Sn based and contains, by weight percentage, 7 to 20% Sn, 0.1 to 5% Pb, 1 to 10% Si, and 0.01 to 1.0% in total of one or more of Cr, Mn, Fe, Ni, Co, Ti, V and Zr, and 0.3 to 3.0% in total of Zn and Mg, as well as 0.01 to 0.1% Sb, with the balance essentially consisting of Al. The dispersed and precipitated Si particles in the matrix are spheroid, oval, or have shapes with rounded ends. Sn—Pb alloy particles precipitate in the vicinity of such Si particles. The hardness of the alloy is slightly less than Hv 40.    Patent Document 4: Japanese Patent No. 3472284 relates to “an aluminum-based bearing alloy, characterized in that: it contains 1.5 to 8 mass % Si, with the balance essentially consisting of Al; the Si particles having a particle diameter of less than 4 μm account for 20 to 60% of the area of the Si particles present on the sliding surface and the Si particles having a particle diameter of 4 to 20 μm account for 40% or more of the area of the Si particles present on the sliding surface.” The latter Si particles having a particle diameter of 4 to 20 μm (40% by area or more) contribute to wear resistance, and the former Si particles having a particle diameter of less than 4 μm (20 to 60% by area) contribute to fatigue resistance. During the wear resistance test, start and stop are repeated. Si particles having a particle diameter of 4 to 20 μm are subjected to the load of an opposite shaft but are neither buried in the Al matrix nor separated from the Al matrix. Wear resistance of the aluminum alloy against S55C, which is the opposite material in the Examples, is improved. Its heat treated state is not described.    Non-Patent Document 1: “Advance Papers of Academic Lecture Meeting of Corporate Juridical Person Automobile Engineering No. 46-99 , 211, Development of Aluminum-Alloy Bearing with Improved Wear Resistance” relates to a research of wear resistance of Al—Sn—Si alloy against an opposite shaft made of quenched steel (Hv 720-850). In the “Discussion” section of this document, the following is described. “The relatively soft matrix of Al alloy is exclusively worn out during the progress of wear. Therefore, Si enriches on the sliding surface. As the amount of Si increases on the sliding surface, much more load can be supported to such an extent that wear of bearing is suppressed. However, when Si particles have a small diameter or hardness is low, Si separates from the surface during sliding, as Si continuously enriches. Consequently, separation and enrichment of Si repeat during the progress of wear such that the Si concentration on the surface remains unchanged. On the other hand, when Si particles have a large diameter and hardness is high, Si is firmly held in the alloy and does not separate from the alloy. In this case, the progress of wear is slow.”
The above notion has led to the development of a bearing alloy which has a composition of Al-4.5Sn-2.7Si-1.5Cu-0.2Cr-0.15Zr and the following features.    (1) Si is included in the aluminum-based bearing alloy as hard matter and has a size of 5.5 μm in terms of average particle diameter. Hardness of the developed material is 53HV. The developed material is wear resistant as high as three to four times the conventional material.    (2) Improved wear resistance is believed to be attributable to the mechanism that the enriched Si is firmly held on the sliding surface and hence bears load.    (3) Structure and strength of alloy having an average particle diameter of 5.5 μm is re-designed, and, as a result, fatigue resistance of the developed alloy under reciprocating load is equivalent to conventional materials, and fatigue resistance under a rotary load is improved as compared with conventional materials. Seizure resistance is improved as compared with a conventional material Al-12.5 Sn-2.7Si-1.8Pb-1Cu-0.2Cr alloy.
The applicants including the present applicant proposed in Patent Document 5: Japanese Unexamined Publication (kokai) No.2004-28278 a bearing apparatus comprising a shaft and a plain bearing. The shaft is a crank shaft made of a non-heat treated steel having a metallographic structure of pearlite or a metallographic structure consisting of 3% by area or less of primary ferrite and pearlite. The steel is neither high-frequency quenched nor surface hardened. The surface roughness of the crank shaft is Rz 0.5 μm or less. The hard matter formed of or comprising Si and having a hardness of Hv 900 or more is dispersed in the Al matrix. The hard matter having a size of 6 μm or less is present on the sliding surface. Cementite having approximately a hardness of Hv 700 and ferrite having a hardness of approximately Hv 150 to 300 are the structural elements of the non-heat treated steel.
The applicants including a present inventor propose in Patent Document No. 6: Japanese Unexamined Patent Publication (kokai) No. 2004-28276 a bearing apparatus comprising a shaft, which is the same as that of Patent Document No. 2. The hard matter in the Al matrix of the Al-based alloy is or comprises Si and has a hardness of Hv 900 or more. The Si content is 1 to 4 mass %.
An Al alloy proposed in Patent Documents Nos. 4 and 5 contains, by mass %, in addition to 2 to 20% of Sn, the following elements: 1 to 3% Pb, 0.5 to 2% Cu, 0.1 to 1% Cr, 0.5 to 2% Mg, and 0.1 to 1% of one or more selected from Zr, Mn, V, Ti and/or B. Si contained in said Al alloy takes the form of hard Si particles having a certain size and smoothens surface of a non-heat treated steel shaft. As a result, wear resistance and seizure resistance are improved.
From the overall review of the prior art references, the following technical level is specified in relation to the Si particles. In the case of a soft opposite shaft or material consisting of non-heat treated steel proposed in Patent Documents Nos. 5 and 6, the Si particles of aluminum alloy have lapping effect on the opposite shaft. In Patent Documents Nos. 1, 2 and 3, the Si particles of aluminum alloy are maintained to have a point contact with the quenched steel of the opposite shaft. Recently, since low viscous ATF has come to be used and various pumps are small-sized, the sliding conditions become severe. Therefore, required levels for wear and seizure resistances are enhanced. The start and stop conditions are employed in the wear test of Patent Document 4, because these conditions are severe and necessary to test enhanced level of wear and seizure. Patent Document 4, however, does not refer to the heat treated state of a steel shaft. Patent Document 4 describes that Si particles are held by the Al matrix during sliding. Non-patent Document 1, which was published around the same time as Patent Document 1, is believed to be the first publication stating that Al, Sn and the like are preferentially abraded by an opposite shaft made of quenched S55C, with the result that Si enriches on the sliding surface.